Optionally store a local copy of (generated) files (caching)

[benoitbryon]

Currently, optimizations are not available for every backend, and virtual files cannot be streamed by reverse proxies.
Moreover, if someone requests the same virtual file twice, contents must be generated again from scratch.
One solution would be to have a cache for generated files. Or for any file, including HTTPFile or remote StorageFile.
The cache would make it possible to avoid re-generation of files.
It also would make it possible to resume a download (see #67) for virtual files.
If the cache is local, then it would allow Apache and Lighttpd to stream remote files (they are currently limited to local files).